Reducing population smoking prevalence is an urgent public health priority that can save more lives and money than almost any other preventive intervention. The Internet offers a promising delivery channel for evidence-based cessation treatment and the potential for enormous public health impact (reach x efficacy). Millions of smokers use the Internet for cessation assistance each year. Studies have shown quit rates of 7-28% across various Internet programs, and a positive relationship between intensity of use and higher abstinence rates. However, most smokers engage minimally with even the best designed cessation websites, diminishing their impact due to limited use of effective treatment components. Poor adherence has been noted across Internet behavior change programs. The ubiquity of mobile devices and their effectiveness in promoting adherence in other areas of health behavior change make them a promising tool to address adherence in web-based interventions. Text messaging is accessible and easy to use; messages can proactively prompt/remind and encourage continued exploration of a web-based intervention; interactive text messaging can facilitate engagement with core Internet intervention components. We have identified 4 aspects of a text message intervention that may enhance its effectiveness in promoting adherence to a web-based cessation program: personalization (yes/no), integration (yes/no), dynamic tailoring (yes/no), and message intensity (standard/decreasing). Phase I of this two-phase study will use a 2-level full factorial design to test the impact of these 4 experimental features on adherence to a web-based intervention. The primary outcome in Phase I will be a composite metric of adherence that incorporates number of logins, time on site, page views, and use of interactive site components shown to predict abstinence. Participants will be N=860 adult smokers that are new registered users of an established Internet cessation program and enroll in its text message program. Based on Phase I, we will optimize our text message intervention to incorporate all experimental features shown to positively impact adherence. Phase II will be a 2-arm randomized trial to compare the efficacy of a web-based cessation program (WEB) alone and in conjunction with the optimal-adherence text messaging intervention (WEB+OA_TXT) on 30-day point prevalence abstinence at 9 months. Phase II participants will be N=600 adult smokers that are new registered users of an established Internet cessation program and enroll in text messaging. We will examine whether 3 month measures of adherence mediate treatment effects on outcome. Improving adherence to proven web-based cessation interventions is critical to leverage the potential public health impact of this broad reach treatment modality. The proposed study is innovative in its use of an optimized text messaging intervention to improve adherence. Study findings will inform adherence research in other areas and will add to the growing knowledge base about the overall effectiveness of Internet cessation programs and mechanisms through which their population impact on smoking prevalence can be improved.